1. Background of the Invention
The present invention relates to a plasma doping method, and in particular, to a plasma doping method for doping an impurity into a surface of a solid sample such as a semiconductor substrate.
2. Description of the Related Art
As a technology for doping an impurity into a surface of a solid sample, a plasma doping (PD) method for ionizing the impurity and doping the ionized impurity into a solid at low energy is well known (for example, see U.S. Pat. No. 4,912,065).
On the other hands, among the methods for doping an impurity, an ion implantation method is most widely used at present. The plasma doping method is described in “Column of Shallow Junction Ion Doping of Figure 30 of Front End Process in International Technology Roadmap for Semiconductors 2001 Edition (ITRS2001)” and “International Technology Roadmap for Semiconductors 2003 Edition (ITRS2003)” as a next-generation technology for implanting ion. The plasma doping method is different from the ion implantation method. Moreover, ITRS is a document that is widely referred to by engineers in semiconductor industries. A technical difference between ion implantation and plasma doping will now be described in more detail.
In the ion implantation method, an apparatus comprising an ion source for generating plasma from gas, an analysis magnet for performing mass separation in order to select desired ions among ions extracted from the ion source, an electrode for accelerating the desired ions, and a process chamber for implanting the accelerated desired ions into a silicon substrate, is used. In the ion implantation, in order to implant the impurity shallow, it is preferable to set extraction energy for extracting ions from the ion source and acceleration energy for accelerating small. However, when the extraction energy is set small, the number of ions to be extracted is decreased. In addition, when the acceleration energy is set small, while an ion beam is transported from the ion source to a wafer, a beam diameter is widened due to a repulsive force generated by charges between the ions. As a result, the ion beam may collide against the inner wall of a beam line, and a large number of ions may be lost. For this reason, throughput of an implantation processing will be lowered. For example, when B+ ions are implanted, if the acceleration energy becomes 2 keV or less, the throughput starts to be lowered, if the acceleration energy becomes 0.5 keV or less, the beam transportation itself become difficult. Further, even though the acceleration energy is lowered to 0.5 keV, the B ions may be implanted at a depth of approximately 20 nm. That is, in case of forming an extension electrode having a thinner thickness than the depth, productivity may be lowered drastically.
In contrast, in the plasma doping method, an apparatus comprising a plasma generation source for inducing plasma into a cylindrical vacuum chamber in which a silicon substrate can be disposed, a bias electrode on which the silicon substrate is disposed, and a bias power supply for adjusting a potential of the bias electrode, is used. The apparatus has the different configuration from the apparatus used in the ion implantation in point that the analysis magnet and the acceleration electrode are not provided. The bias electrode serving as a plasma source and a wafer holder is provided in the vacuum chamber. Then, the ions are accelerated and introduced by a potential to be generated between the plasma and the wafer. With this configuration, since low-energy plasma can be directly used, a large amount of low-energy ions can be irradiated onto the wafer, compared with the ion implantation. That is, a dose rate is considerably high. For this reason, in the low-energy B-ion implantation, high throughput can be kept.
By applying the plasma doping method, the inventors have developed a process technology for forming a source-to-drain extension electrode having a very shallow thickness and low resistance. The paper on this new process technology is adopted in VLSI Symposium that is the highest authority as one of International Conferences, in June 2004. This new process technology is known as a process technology that has particular effects (“Y. Sasaki, et al., Symp. on VLSI Tech. p180 (2004)”).
In this method, doping material gas which is introduced from a gas introduction port, such as B2H6, is plasmized by a plasma generation unit having a microwave waveguide and an electric magnet. Then, boron ions in plasma are supplied to a surface of a sample by a high-frequency power supply.
With the reduction in size and high integration of a semiconductor device, characteristics in an impurity doped region are very important. Among these characteristics, a dose (impurity doping amount) determines low resistance that is one of important elements in determining element characteristics. Accordingly, the control of the dose is very important.
If the plasma doping method is used, it can be seen that the source-to-drain extension electrode having a very shallow thickness and low resistance can be formed. However, a dose control method for controlling the element characteristics has not been developed yet. Up to now, a method for changing the dose by way of changing a plasma doping time has been tested, but this method does not obtain sufficient control precision, and as a result, it is unpractical.
In this situation, as a method which is capable of improving safety by diluting toxic B2H6 having a serious risk to the human body as large as possible, stably generating and keeping plasma without degrading doping efficiency, and easily performing the control of the dopant dose, the inventors has suggested the following method. In the method, B2H6 gas as a material containing an impurity to be doped is diluted with He gas having small ionization energy, then He plasma is generated earlier, and subsequently B2H6 is discharged (JP2004-179592) suggested that the concentration of B2H6 gas is preferably less than 0.05%.
When the concentration is low, for example, approximately 0.05%, although it is reported that the dose is easily controlled, changing the dose by varying the time while the gas concentration is kept constant is described. That is, when the B2H6 gas concentration is low, the change in the dose is small with respect to the variation in time, and thus the dose is easily controlled. Here, there is a progress in that the control precision of the dose is increased. However, this just improves the known method for changing the dose by changing a plasma doping time. Herein, there is no study for the relationship between the change in the dose and the gas concentration.
As described above, it is important to form the impurity doped region or control the dose. Of course, in-plane uniformity is also very important to form an element. In particular, while there is a recent progress in a large diameter of a wafer, it is very difficult to obtain a uniform dose in the surface.